multifilesrc: fix seeking support

Submitted by Jorge

I build a pipeline like this:

multifilesrc ! jpegdec ! ffmpegcolorspace !videosink

(where multifilesrc has a "image/jpeg,framerate=15/1" cap for correct playing)

I can make it play/pause/stop, but it won't seek nor allow query for a position. Using from pyhton:

pos = self.player.pipeline.query_position(gst.FORMAT_DEFAULT, None)[0]

returns the following error:

gst.QueryError: query failed

And doing

self.player.pipeline.seek_simple(gst.FORMAT_DEFAULT, gst.SEEK_FLAG_FLUSH, 3)

does nothing (nor error, nor seeking). From trying stuff it seems as if multifilesrc only respects it's "index" property.